Scanning electron microscope

ABSTRACT

Disclosed is a scanning electron microscope which comprises the means for periodically changing the focal distance of the final condenser lens at two stages and the means for simultaneously displaying two scanning images corresponding to the two focal distances.

United States Patent 1191 Suganuma 1 July 24, 1973 SCANNING ELECTRON MICROSCOPE 3,502,870 3/1970 Fujiyasu 250 495 E 3,504,176 3/1970 Than 250/496 D [751 Invent fl' h' 3,509,275 4/1970 Deeley 250/495 A y Japan 3,614,311 10/1971 Fujiyasu 1 250/495 A [73] Assignee: Nihon Denshi Kabushiki Kaisha, 3,585,382 6/ 1971 Suganuma 250/495 E Tokyo, Japan 7 Primary Examiner-James W. Lawrence [22] Flled' Sept 1971 Assistant ExaminerC. E. Church [21] Appl. No.: 178,314 Attorney-John M. Webb [52] U.S. Cl. 250/495 A, 250/495 D 57 ABSTRACT [51] Int. Cl. H01] 37/26 581 Field of Search 250/495 A, 49.5 E, Dlscbsed scanmng electm" whwh 250/495 D 49.5 PE comprises the means for periodically changing the focal distance of the final condenser lens at two stages [56] References Cited and the means for simultaneously displaying two scan- UNITED STATES PATENTS ning images corresponding to the two focal distances. 2,973,433 211961 Kramer 250/495 D 15 Claims, 17 Drawing Figures ELECTRON GUN HORIZONTAL VERTICAL SCANNING SCANNING GENERATOR GENERATOR 0.0. SOURCE SWITCHING l NAL GENERATOR S G 13%; 3 iii 1,

POWER PATEIIIEIIII I 3.748.467

SHEEN; BF 4 ELE TRON mm 3 6/, 6r I-IORIzoNTAL vERTIcAL ScANNING m- SGANNING GENERATOR GENERATOR V l 5 40.0. BIAS SOuRcE 7 CIRCUIT\,\ 9 R f L /z SWlTCH 26 SIGNAL lXIZSl A AMPLIFIER -STIGMATOR' POWER ELECTRON GUN \T Z 6r HORIZONTAL j VERTICAL SCANNING SCANNING 5V GENERATOR GENERATOR 4? a LI- L SWITCHING SIGNAL E D-C-POWER GENERATOR #6 M? m /0 2K A /4r AMPLIFIER STIGMATOR SCANNING ELECTRON MICROSCOPE BACKGROUND OF INVENTION This invention relates to a scanning electron microscope, and, in particular to a device which facilitates the correction of axially asymmetric astigmatism of the final condenser lens in a scanning electron microscope.

DESCRIPTION OF PRIOR ART The resolving power of the scanning image obtained by a scanning electron microscope is essentially determined by cross-section of an electron probe irradiated on a specimen. It must be as small as possible. In order to meet this requirement, as is already known, axially asymmetric astigmatism of the final condenser lens should be corrected. This asymmetric astigmatism is essentially caused by the nonuniformity of material of the lens magnetic pole piece, incompleteness of mechanical work and contamination of the aperture. To correct asymmetric astigmatism, generally a stigmator is employed whereby the direction and intensity of the quadrupole lens magnetic field is controlled. However, in observing a scanning image, it is very difficult to determine whether there is astigmatism in the image or not. Therefore, adjustment of the stigmator requires much skill and time and still less than satisfactory results are obtained.

SUMMARY OF INVENTION Briefly, according to this invention, a scanning electron microscope comprises a means for periodically changing the focal distance of a final condenser lens at two stages and a means for displaying the images corresponding to the said two focal distances.

Other objects and advantages of this invention will become apparent to those skilled in the art from the following description made in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagrammatic view showing one embodiment of the invention;

FIGS. 2a 2e explain the operation of the device as shown in FIG. I;

FIGS. 3, 4a 4c and 5a 5c illustrate the extent to which astigmatism of the final condenser lens influences the scanning images; and,

FIGS. 6, 7, 8 and 9 are diagrammatic views showing other embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, the electron optical system 1 of a scanning electron microscope comprises an electron gun 2, a first condenser lens 3, a second condenser lens 4 (final condenser lens), and deflecting coil 5V for scanning an electron probe on a specimen in the vertical direction. The scanning signal generator 6V supplies the said coil 5V with a sawtooth current. The deflecting coil SR is for scanning an electron probe on a specimen in the horizontal direction. The scanning signal generator 6H supplies the said coil 5H with a sawtooth current. The output of the said scanning signal generator 6H is shown in FIG. 2(a).

Switching signal generator 7 generates a pulse signal that is synchronized with the horizontal scan signal shown in FIG. 2 (a). The output of the generator 7 is shown in FIG. 2 (b). Power source 8 generates a positive or a negative DC. current corresponding to the said pulse signal. The output of source 8 is shown in FIG. 2 (c) and is suppliedv to auxiliary lens coil 9 which slightly changes the focal distance of the final condenser lens. As a result, the focal distance changes to f or f, at every switching signal pulse shown in FIG. 2 (b). The signal (e.g., an X-ray, a secondary electron, a reflected electron, a transmission electron, etc.) from specimen I0 irradiated by the electron probe is detected by a detector 11, amplified by amplifier circuit 12, and thereafter applied to control grids 13c and Me of cathode ray tubes 13 and 14. On the other hand, a part of the output signals of scanning signal generators 6V and GB is supplied to deflecting coils 13V, 13H, 14V and 14H, which deflect the electron beam in the cathode ray tube. Thus, a brightness modulation scanning image appears on the screen of the cathode ray tube. Electric potentials of control grids 13b and 14b are provided from blanking signal generator 15, which is controlled by switching signal generator 7 so that the brightness modulation scanning line is displayed alternately on the screen of cathode ray tube 13 or on the screen of cathode ray tube 14. The potential of control grid 13b is shown in FIG. 2 (d). In this figure, a negative potential sufficient to cut off the electron beam during the term when the focal distance is f, is maintained as the grid potential, and the potential of control grid 14b is shown in FIG. 2 (e). In this figure, a negative potential sufficient to cut off the electron beam during the term when focal distance f, is maintained as the grid potential. It will be noted from the above that cathode ray tube 13 displays a scanning image only when the focal distance of the final condenser lens is f and that cathode ray tube 14 displays a scanning image only when the focal distance of the final condenser lens is f,. FIG. 3 is an explanatory view of the electron probe on the surface of specimen 10 in connection with the focal distance of final condenser lens 4. In the view, the Z-axis shows an optical axis of the final condenser lens; xy axial plane perpendicular to Z-axis shows a position of the principal plane of the final condenser lens. Focusing position 18 of the electron beam 171: incident on the xz plane is not in agreement with focusing position 19 of the electron beam 17y incident on the yz plane due to axially asymmetric astigmatism. Both beams 17X and 17Y are electron beams which come from image point 16 focused at the image position of the first condenser lens. In this case, at the position 20, a circular cross-section (circle of least confusion) of the electron probe is obtained. The said cross-section is always larger than the minimum probe cross-section obtained when axial asymmetric astigmatism is not present.

Now, final condenser lens 4 is adjusted to achieve the focal distance by f, in which the cross-section of the electron probe on the surface of the specimen forms the circle of least confusion as described above. From this condition, if the focal distance of the final condenser lens is increased and decreased to attain f and f, respectively, the cross-sections of the electron probe on the surface of the specimen will become ellipses, the longitudinal axis of which will cross each other at right angles. If a scanning image of the circular shaped portion of the specimen is scanned by the electron probe having an elliptically shaped cross-section as described above, scanning images shown in FIG. 4 will be obtained. FIG. 4 (a) shows an image at focal distance f,. This image is remarkably out of focus in the direction of the longitudinal axis of the said elliptical electron probe. FIG. 4 (b) shows a properly focused image at focal distancef FIG. 4 (c) shows an image at focal distancef the image being remarkably out of focus in the direction of the longitudinal axis of the elliptical electron probe. So, the images of FIG. 4 (a) and FIG. 4 (c) are different from each other. The depth of undistorted focus is very narrow. This phenomenon arises when the axial astigmatism of the final condenser lens is present. When astigmatism is not present or has been completely corrected, the phenomenon never arises even if the focal distance of the final condenser lens is varied. In this condition, the difference of images is shown in FIG. 5. FIG. 5 (a) and FIG. 5 (c) show images of circular shaped portions in the specimen at focal distances f and f FIG. 5 (b) shows an image at focal distance f- Normally, astigmatism is corrected by means of the stigmator. In FIG. 1, a coil for generating a magnetic field for correcting astigmatism is shown by 21 and its power source by 22. An ordinary stigmator is required to make both adjustments of the direction and intensity of its magnetic field. And, at the same time, it is necessary to know whether there is any astigmatism. To this end, heretofore, an operator had to keep in mind the image obtained at a certain lens focal distance, and had to compare the said image with the image obtained by manually changing the lens focal distance and then distinguishing what differences if any exist between the said two images. On the other hand, in the device shown in FIG. 1, scanning images are displayed on the screens of cathode ray tubes 13 and 14 corresponding to the respective focal distances f and f Thus, complete correction can be made by comparison of the two scanning images and by adjustment of the stigmator so as to eliminate the difference in the above-compared scanning images. Therefore, adjustment of the stigmator can be made readily, rapidly and accurately.

FIG. 6 illustrates, by way of a diagram, another embodiment of the present invention. The embodiment, according to the invention, is not provided with an auxiliary exciting coil slightly changing the focal distance of the final condenser lens. Instead, bias circuit 24 is provided to occasion very small changes in the output of power source 23 for supplying the exciting current into the final condenser lens. This circuit is so designed that it is controlled by a signal from switching signal generator 7.

Still another embodiment of the invention is shown in FIG. 7. This embodiment is distinguished from the device of FIG. 1 in that it does not employ grids 13b and 14b for blanking in cathode ray tubes 13 and 14. The device of the embodiment is provided with a circuit 25 which alternately supplies grids 13c and 14c with cut off voltages corresponding to the signal from the circuit 7.

FIG. 8 shows another embodiment of the invention. In this embodiment, biasing circuit 26 is placed between horizontal scanning signal generator 6H and horizontal deflecting coil 13H of cathode ray tube 13 so that two different biasing voltages are alternately superimposed on the output voltage applied from horizontal scanning signal generator 6H to horizontal deflecting coil 13H of cathode ray tube 13. The said circuit 26 is synchronized with the operation of switching signal generator 7. As a result, the electron beam of cathode ray tube 13 alternately scans the left-hand half,

A, and the right-hand half, 8, of the screen of cathode ray tube 13. According to such an arrangement, two images of different focal distances f and f can simultaneously be displayed on the left-hand half, A, and the righthand half, B, of the screen of cathode ray tube 13.

FIG. 9 shows still another embodiment according to the invention. In this embodiment, switching signal generator 7 is synchronized with the vertical scanning signal from generator 6V so that the said generator 7 generates a pulse signal at the end of each scanning frame of cathode ray tube 14. Accordingly, the embodiment can alternately display the scanning images corresponding to two focal distances of the final condenser lens on the screen of single cathode ray tube 14.

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

I claim:

1. A scanning electron microscope comprising a means for generating an electron beam, an electron lens for focusing the said electron beam on a specimen, a deflector for scanning the electron beam over the specimen, a detector for detecting the signal emitted by irradiating the electron beam on the specimen, a stig mator for correcting astigmatism of the said electron lens, the improvement comprising means for adjusting said stigmator including means for changing the focal distance of the electron lens automatically and periodically between two selected distances, and means for displaying two scanning images corresponding to each of said focal distances.

2. A scanning electron microscope according to claim 1 wherein two cathode ray tubes comprise the means for displaying scanning images corresponding to each focal distance separately and individually.

3. A scanning electron microscope according to claim 2, comprising means for alternately providing a cut-off potential to the control grids of either one of the cathode ray tubes, the means being synchronized with the lens focus.

4. A scanning electron microscope according to claim 2, wherein the lens focus changing means includes a switching signal generator, a power source for alternately generating negative or positive D. C. currents according to the output signal of the said generator, and an auxiliary lens coil excited by the said power source.

5. A scanning electron microscope according to claim 2, wherein the lens focus changing means includes a switching signal generator and a means for changing the exciting current of the electron lens between two selected distances according to output signal of the said generator.

6. A scanning electron microscope according to claim 4, wherein the switching signal generator synchronizes with the deflector for the scanning electron beam.

7. A scanning electron microscope according to claim 1, wherein one cathode ray tube displays different scanning images on different areas of its screen corresponding to the two focal distances.

8. A scanning electron microscope according to claim 7, comprising means for supplying a deflecting current to the deflecting coil of the cathode ray tube, the deflecting current consisting of a horizontal scanning signal current and of a D. C. bias positive or negative signal current synchronized with the said horizontal scanning signal.

9. A scanning electron microscope according to claim 7, wherein the lens focus changing means comprises a switching Signal generator and means responsive to the output signal of the said generator for changing the exciting current of the electron lens between two selected distances.

10. A scanning electron microscope according to claim 7, wherein the lens focus changing means comprises a switching signal generator, a power source for alternately generating negative or positive D. C. current according to the output signal of the said generator, and an auxiliary lens coil excited by the power source.

11. A scanning electron microscope according to claim 9, wherein the switching signal generator is synchronized with the deflector for the scanning electron beam.

12. A scanning electron microscope according to claim 1, wherein the means for displaying the scanning images provides two scanning images alternately displayed at time intervals on the screen of a single cathode ray tube.

13. A scanning electron microscope according to claim 12, wherein the lens focus changing means comprises a switching signal generator and a means responsive to the output signal of the generator for changing the exciting current of the electron lens between two selected distances.

14. A scanning electron microscope according to claim 12, wherein the lens focus changing means comprises a switching signal generator, a power source for alternately generating negative or positive D. C. current according to the output signal of the said generator, and an auxiliary lens coil excited by said power source.

15. A scaning electron microscope according to claim 13, wherein the switching signal generator is synchronized with the deflector for scanning the electron probe.

UNITED- STATES PATENT OFr 10E CERTIFICATE OF CORRECTIGN Patent No. 3, 748, 467 Dated July 24, 1973 Inventor Wm liege .fieea -7 .lVL is cert 1 [led that error appears in the above-Idem. I 1' led puLcnL and that said Letters Patent are hereby corrected as shown below:

--Foreign Application Priority Data 7 September 11, 1970 Japan 45-79798-- 1 Signed and sealed this 6th day of August 1971.

(SEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN Commissioner of Patents Attesting Officer 

1. A scanning electron microscope comprising a means for generating an electron beam, an electron lens for focusing the said electron beam on a specimen, a deflector for scanning the electron beam over the specimen, a detector for detecting the signal emitted by irradiating the electron beam on the specimen, a stigmator for correcting astigmatism of the said electron lens, the improvement comprising means for adjusting said stigmator including means for changing the focal distance of the electron lens automatically and periodically between two selected distances, and means for displaying two scanning images corresponding to each of said focal distances.
 2. A scanning electron microscope according to claim 1 wherein two cathode ray tubes comprise the means for displaying scanning images corresponding to each focal distance separately and individually.
 3. A scanning electron microscope according to claim 2, comprising means for alternately providing a cut-off potential to the control grids of either one of the cathode ray tubes, the means being synchronized with the lens focus.
 4. A scanning electron microscope according to claim 2, wherein the lens focus changing means includes a switching signal generator, a power source for alternately generating negative or positive D. C. currents according to the output signal of the said generator, and an auxiliary lens coil excited by the said power source.
 5. A scanning electron microscope according to claim 2, wherein the lens focus changing means includes a switching signal generator and a means for changing the exciting current of the electron lens between two selected distances according to output signal of the said generator.
 6. A scanning electron microscope according to claim 4, wherein the switching signal generator synchronizes with the deflector for the scanning electron beam.
 7. A scanning electron microscope according to claim 1, wherein one cathode ray tube displays different scanning images on different areas of its screen corresponding to the two focal distances.
 8. A scanning electron microscope according to claim 7, comprising means for supplying a deflecting current to the deflecting coil of the cathode ray tube, the deflecting current consisting of a horizontal scanning signal current and of a D. C. bias positive or negative signal current synchronized with the said horizontal scanning signal.
 9. A scanning electron microscope according to claim 7, wherein the lens focus changing means comprises a switching signal generator and means responsive to the output signal of the said generator for changing the exciting current of the electron lens between two selected distances.
 10. A scanning electron microscope according to claim 7, wherein the lens focus changing means comprises a switching signal generator, a power source for alternately generating negative or positive D. C. current according to the output signal of the said generator, and an auxiliary lens coil excited by the power source.
 11. A scanning electron microscope according to claim 9, wherein the switching signal generator is synchronized with the deflector for the scanning electron beam.
 12. A scanning electron microscope according to claim 1, wherein the means for displaying the scanning images provides two scanning images alternately displayed at time intervals on the screen of a single cathode ray tube.
 13. A scanning electron microscope according to claim 12, wherein the lens focus changing means comprises a switching signal generator and a means responsive to the output signal of the generator for changing the exciting current of the electron lens between two selected distances.
 14. A scanning electron microscope according to claim 12, wherein the lens focus changing means comprises a switching signal generator, a power source for alternately generating negative or positive D. C. current according to the output signal of the said generator, and an auxiliary lens coil excited by said power source.
 15. A scaning electron microscope according to claim 13, wherein the switching signal generator is synchronized with the deflector for scanning the electron probe. 